Samsung SGH-2488 Service Manual

Also see for SGH-2488: Owner's manual

Contents

Samsung SGH-2488 Service Manual Manual pdf 5 page image

Samsung Electronics 2-12. Circuit Description2-1 RF Part2-1-1 Frequency GeneratorThe 13MHz reference clock (VCTCXO) drives the logic and RF part. The 13 MHz reference is controlled by thelogic (10bits DAC minimum) and is kept to a frequency error less than ±0.1 ppm after synchronization with theGSM network.The IF VCO generates 233~259.5MHz VHF LO used in the RX I,Q Demodulator and afterdoubling(493~519MHz), it used in the TX I,Q modulator. The UHF LO for the first RX down conversion andthe TX offset mixing works in superheterodyne mode to reduce the relative bandwidth and to be able to workat a frequency greater than 1 GHz.2-1-2 TransmitterThe baseband GSM chipset (Kernel 5) generates I and Q baseband signals for the transmit modulator. Themodulator (U2893B) is the first stage which takes the baseband signal and upconverts it to a fixed intermediatefrequency. The modulator provides more than 35dBc of carrier and unwanted side-band refection and producesGMSK modulated signal, the “reference” signal at 493~519 MHz which passes to the offset phase-locked loopblock (OPLL). The OPLL consists of a down-converter, phase detector, loop filter and transmit VCOs operationat the final RF output frequency. The down converter mixes the UHF LO (eg. 1210MHz) with the transmit VCOsignal to generate a “feedback” signal at 493~519 MHz. The “feedback” signal passes via a limiter to one portof the phase detector. The GMSK “reference” signal from the modulator passes via second limiter to the otherinput port of the phase detector. The phase detector generates an error current proportional to the phasedifference between the “feedback” signal from the down-converter and the “reference” signal from themodulator. This error current is filtered by a third order low-pass filter to generate an output voltage whichdepends on the GMSK modulation and the desired channel frequency. This voltage controls the transmit VCOsuch that the VCO output signal, centered on the correct RF channel, is frequency modulated with the originalGMSK data. The center frequency of the transmit VCO is offset from the UHF LO frequency by 493~519MHz.The OPLL acts as a tracking narrowband band pass filter tuned to the desired channel frequency. This reducesthe wideband products. The OPLL architecture results in a low-noise GMSK modulated signal at902MHz(1747MHz) with very low spurious content.The RF GMSK output from the transmit VCO is fed to the RF power amplifier. The peak output power and theprofile of the transmitted burst are controlled by means of a closed feedback loop. The RF output from the PA issampled with a directional coupler. The sampled signal passes to an RF detector diode whose output voltage isdependent on the incident RF level. This “feedback” voltage passes to the inverting input of the loop integrator.A “reference” signal is generated within the baseband section under control of the layer 1 software. The loopmaintains zero difference between the “feedback” signal and the “reference” signal. In this way, the amplitudeand shape of the transmitted RF burst may be controlled by the baseband processor. In particular, the rise andfall profiles can be controlled to meet the stringent power/time templates and switching transient requirementsof GSM 05.05.The RF output passes to the antenna connector via an integrated TX/RX switch and lowpass filter to attenuatethe harmonics generated by the power amplifier.